Monday, March 14, 2016

Box your Battery

Tired of lugging a heavy battery around by its carrying strap? Inconvenienced by connecting to it using alligator clamps? Messed up by dew getting the battery all wet? Make a battery box!

It's simpler than you might think and you can make it as fancy as you want. Here's my rather minimalist box:

A Simple Battery Box

At the right side under the lid protuberance designed to allow dew to run off are two 12V sockets. I may add a couple more on the far side. Each is individually fused so that if one device blows the rest keep working.
The voltmeter springs into action
The only other thing I added was an LED voltmeter that's actuated by a momentary-on push button. This helps me monitor the battery's state of charge.

Under the hood
The 35Ah battery (group U1) is dwarfed by the box, which is designed to handle batteries in groups 24-31. I've used rigid foam to hold it in place. I haven't done a deep recharge of the battery yet, so I don't know how warm it will get. I suspect it won't get warm at all--My charger is only 1.1A. With an accessory the charger can charge the battery though one of the sockets

What you'll need:
  • The box: The one I used comes with a useful divider and can be used with a  battery that provides 75Ah and still have room for the sockets
  • Sockets are found on eBay for about $6 each. I used waterproof ones that come with their own cables and fuses. I bought the kind that attach with a locking ring; they've been solid in use.
  • The voltmeter (about $4) is also from eBay, waterproof, and also attaches with a ring.
  • Connecting wire. If you run your own wire to the sockets, be sure to use 14AWG or heavier wire, and check that the fuses are appropriate for the wire and devices you plan to connect. Wire to the voltmeter can be much lighter as it will carry only a tiny current.
  • Tools: The only helpful tool you may not have is a variable drill bit that will help make the socket and meter holes. You don't need one as large as the holes--it can be used the start the hole and ream it to proper size.
What you don't need:
  • Battery terminals for jumper cables. This should be a deep-cycle battery, and it doesn't like jump starting. 
  • A carrying strap. The box comes with handholds at either end. A 35Ah battery and box weighs under 25 pounds, so it's easy to carry.
As you can see from the above picture a U1-sized battery leaves a lot of room for accessorizing. An amp meter could be useful to install if you want to measure battery drain. If you have the need for other voltages, add some step-down DC converters. They're small and very inexpensive. You should probably isolate them from any charger, perhaps with a two-way rocker switch. Go wild and add lights and a radio and you have one of the overpriced commercial "power tanks."

I'll be field testing this battery over the summer at a number of star parties, and may modify it as a result.

My battery box delivers twice the watt hours of a commercial "power tank," and costs less (about $125 total,  $100 of which is the battery and charger).

Friday, March 11, 2016

Trying My New Battery; Color Balance Issues

The weather has been cooperating a little better this last week, and I've been able to try out my battery-based imaging setup. I haven't done much more than image an hour here, a couple of hours there, but the results are about as expected.

Here are two images from my inner red zone back yard,


As you can see I was unable to completely eradicate the sky brightness gradient.

The Rosette:

Both are taken without filters only because I don't have one. A decent filter would have improved these greatly.

Here are a couple from some distance north of the Twin Cities in a yellow zone,

75m of the M81/82 area:

I have this delusion that I'll be able to image the integrated flux nebula.

The southern portion of Auriga (including some dark lines from tree branches):

The first two images came out horribly red/magenta heavy, and it was a lot of work getting the color right. I passed that off as a consequence of the camera mod and light pollution. The second two appeared just as red on the display screen while working on getting the exposure right, but they came out fairly neutral because I chose the ImagesPlus RAW conversion that ignores the camera's white balance; I used "Bayer No White Balance" instead of "At Capture White Balanced Color."

Color balance isn't an issue when CCD imaging, particularly if your filters are balanced by the manufacturer to give equal white signals in each channel.

The question is now which is better, a custom white balance or using the "no white balance" processing option. There's only one way to find out, and that will require a sunny day, a few sheets of white printer paper, and a clear night. That's my next task.

Oh, and the battery worked perfectly. It's nice to have the power right there in such a light package. 35Ah, less than 23 pounds. My little battery case now has two power sockets, but the volt meter has yet to arrive.

Thursday, March 3, 2016

The Lust for Power, Part 3: Generators

Last time I found two imaging configurations that called for the use of big, heavy, and expensive deep cycle batteries. The common alternative is to buy a generator; it will supply 110V AC, and then your AC adapters will feed your equipment.

Generators as the primary power source

Generators come in a variety of sizes. The factors to consider, roughly in order of their importance, include:
  • Amperage rating (AC). While some generators have DC outputs, you'll probably be using the AC side.
  • Running and peak wattage (W). Wattage is easily computed by taking the product of amps times volts. [Energy is watt-hours (Wh); A battery's available Wh is just the product of its Ah and voltage. For example, a 100Ah 12V battery has a capacity of 1200Wh, of which about 60% is available before recharging.] 
  • Sound level. If you're at a remote site, chances are you'll have people camped nearby. Running a loud generator could get you booted out.
  • AC regulation. Is the ouput voltage well regulated, and is it in the form of a relatively noise-free sine wave? This is difficult information to come by.
  • Available low-energy modes. Does it reduce fuel consumption and sound level if the demand is small?
  • Subjective things like ease of use, noise level, fuel consumption, reliability, etc. Generally the more you pay the more you get in terms of these.
What kind of power will we need? Configuration 1 (CCD, guided large telescope) required 6A. At 12V this is only about 72W. Peak amps is about twice that and demands 150W. Configuration 2 has an even smaller demand (130W peak).

The smallest generators have about 800W peak power and a price point around $300. The PortaSource IG800W ($313) has marginal amps; the Generac ix800 ($287) gets mediocre reviews and a lot of 1-star votes on Amazon.

Medium-capacity generators have around 1600 to 2000W running power and twice the amperage that I require. Prices range from $400 to $600, although some brands can demand about $1000. It's not clear that the added cost is worth it. These generally weigh 50 pounds or more.

In this class I think the Wen 56200i ($429) fits my needs best .

Generators with greater capacity are not needed for imaging, but can also serve as emergency generators. I'm not going to consider them. 

Generators to recharge batteries

It may make some sense to get a small generator and then use it to recharge a battery. because the recharge takes place during daytime noise is relatively unimportant. What matters is the amperage of the charger, since that will be "restocking" the Ah lost overnight. Because some chargers operate at lower amps than your imaging use, you may be able to get by with a generator that doesn't meet specs as the primary power supply.

The required charging time will be the number of hours you imaged  times the amps used while imaging divided by the charger's charging amps.

Example: Imaging at 6A for four hours and using a 1.1A charger. The recharge time is 4h x 6A / 1.1A, or almost 22h. The most you would probably recharge for would be 12h, so you could recover only 13Ah of the 24 you used the night before. Not practical.

Now imagine imaging at 2.9A for four hours and the 1.1A charger. Recharge time is now 4h x 2.9A / 1.1A, or  10.5h. That's actually doable.

I've used a relatively slow charger in this example. A 3.5A charger would cut these times by 2/3 , to 7 and 3.5 hours. A 50Ah battery and 800W generator could make a nice tandem.

Generators to Augment Batteries

Okay, why not power things from both a battery and small generator, thereby easing the power demands on both, then using daytime to top off the battery? Let's see some examples.

Let's consider Configuration 2, small telescope, CCD, guiding, dew prevention: 5.2A. the two big power consumers are the laptop and CCD. Let's let the generator power the laptop and mount (3A), the battery handle the CCD and dew prevention (2.2A). 3A is easily within reach of the small generators, and the nightly Ah drawn from the battery is 8.8Ah. Recharge time is 8 hours. Suppose it's fall and you can image for 6 hours; you'll pull 13.2 Ah out of the battery and need 12 hours to put it back. The generator will run a lot--18 hours a day.

The downside is that nighttime running of the generator brings noise back into consideration. And you'll be running the generator quite a bit--both during imaging and the daytime. It's an interesting approach to powering your gear, but I think either battery or generator is better than a hybrid solution.